Soil nails are a form of earth retention that utilizes grouted bars installed into the face of an excavation. The Excavation is typically taken down in five foot vertical lifts and soil nails are installed at a typical 5-6’ on center spacing. Upon completion of the soil nails a reinforcement element, either a wire mesh or rebar, is installed and attached into the already installed soil nail. For drainage purposes a 2-4’ wide strip of geo-composite drain board is applied to the excavated surface behind the reinforcement. A 4”-6” shotcrete layer is then applied to the cut face of the excavation which ties the soil nails and reinforcement together to create a composite system. The soil nail wall continues down in the same process until it terminates at the base of the excavation.
Soil nail walls can be designed as either a permanent or temporary shoring solution and are very versatile in application. Due to the smaller sized equipment used to install the system they can be installed in tight areas where conventional shoring systems may not be feasible.
Not only are soil nail systems a solution for shoring excavations they can also be utilized for landslides, slope stabilizations, and cut backs into hillsides. Often times when a soil nail system is used to cut back into a hillside the soil nail wall and shotcrete ius used in conjuction with a permanent 6-8” wall that combines with the soil nail system to form a 12-16” permanent wall. This wall can be architectural faced with the second layer of shotcrete to meet the needs or aesthetics of the surroundings.
Soldier Beams & Lagging
The soldier beam and lagging is the most conventional earth retention system used consisting of steel piles and horizontally installed timber lagging. Soldier beam walls are typically installed at 6-8’ intervals around the perimeter of an excavation. Two different methods can be employed to install the beam in the ground. The beams can either be dropped into a drilled shaft and backfilled with concrete or vibrated into place. The installation method used depends on soil conditions, water table, and job site restrictions. Once the beams are in place excavation can begin in 5’ lifts until the base of the excavation is reached. Horizontal timber lagging is installed behind the face of the beams to support the earth being retained.
This system as described is considered a “cantilevered” shoring system. A cantilevered shoring system has its limitations and cannot retian much more than 10-12’ of earth without the addition of lateral support elements. These lateral support elements would be either tieback anchors or a sort of bracing.
Tieback anchors consist of either a bar or strand element installed in a 4” or 10” hole drilled hole. Tieback anchors are typically used in conjunction with earth retention systems and installed in a horizontal fashion at a 15-45 degree decline from plane. Once installed into the hole the hole is backfilled with a grout. When grout has reached strength the tieback is anchored to either a beam or wall and tensioned with the use of hydraulic jack. This act of tensioning creates a load that helps resist earth pressures for shoring systems or mitigate movement in a wall. The number of tiebacks as well as size and length is a function of the demands required on the system.